ABSTRACT
Rocks in the Eastern Vermont Sequence are highly metamorphosed and
are said to range in age from Precambrian to Siluro-Devonian. The
sequence outcrops on Keyes Mountain, about 6km Northwest of
Felchville, Vermont, and an area on this mountain about 2km square
was mapped in great detail.
The dominant rock type found is quartzofeldspathic gneiss, but
schistose gneiss, amphibolite, quartzite and schist also occur.
Most of these rocks are layered but layering has been transposed
in many places and much of it is probably secondary. Layering and
schistosity both dip dominantly to the north at about 25º.
A number of mesoscopic structures can be mapped, including folds,
schistosity, lineations, boudinage and low-angle dislocations.
Folds can be divided into three style groups
- S.G. 1 : tight to isoclinal folds in layering with no
axial-surface foliation.
- S.G. 2 : open to isoclinal folds in layering and schistosity
with weakly to strongly developed axial-surface foliation.
- S.G. 3 : open to tight folds in layering and schistosity with no
axial-surface foliation. Axial surfaces are usually at a high
angle to foliation.
On the basis of overprinting relationships, at least three
generations of folds exist. First and second generations cannot be
separated by style and are referred to as "early folds" ; most
early folds are isoclinal. The third generation includes all S.G.
3 folds, although it is possible that more than one generation is
present in S.G. 3. They are therefore referred to as "late folds".
Mesoscopic late folds plunge dominantly at 25º to 030, their axial
surfaces dip steeply to the west. A large, open macroscopic late
fold occupies the whole area and is probably similarly oriented to
mesoscopic late folds. Mesoscopic early fold axes are
redistributed on a partial small circle by late folding but are
inferred to have had a strong preferred orientation in the
foliation plane before this event.
Evidence offered by previous workers for the interpretation of the
Chester and other domes in Eastern Vermont as "Christmas Tree"
folds is considered to be inadequate. An alternative model
involving refolding of early folds by doubly-plunging late folds
is favoured. Five units with boundaries parallel to schistosity
were delineated, each with distinctive lithologic and structural
characteristics. The structurally "highest" and "lowest" both fit
published descriptions of "basement." The central three units are
characterised structurally by widespread transposition, intensely
developed schistosity, and an abundance of low angle dislocations,
boudinage and isoclinal folds. They are interpreted as a
dislocation zone across which large relative displacements may
have occured.
The stratigraphic formations mapped by previous workers could not
be recognized in this area ; all unit boundaries are thought to be
structural ones and each unit is probably a structural rather than
a stratigraphic entity. It is possible that this is true of other
"stratigraphic" formations and boundaries in the Eastern Vermont
sequence.
Nisbet, B.W., 1976. Structural studies in the northern Chester
Dome of East-Central Vermont. Unpublished PhD dissertation,
State University of New York at Albany. 167pp., +ix; 5 folded
plates (maps)
University at Albany Science Library call number: SCIENCE
MIC Film QE 171 N57X
Copies of this PhD dissertation can be ordered
from Proquest UMI
Front matter (title,
table of contents, abstract, acknowledgements) 0.3MB pdf
file
Photo pages in dissertation
(greyscale
photos
with captions) 13.6MB pdf file
Plate 1 is the same 1:2,400
scale topographic base map used for plate 2
Plate 2 - (coloured outcrop
map, scale 1: 2,400) - pdf file 12 MB
Geological
outcrop
map of the Keyes Mountain area, Ludlow Quad., Vermont
Plate 3 - (structural data
map, scale 1: 2,400) - pdf file 5.4 MB
B3
folds
and layering, Keyes Mountain area
Plate 4 - (structural data
map, scale 1: 2,400) - pdf file 5 MB
Orientation
data
for early folds, Keyes Mountain area
Plate 5 - coloured
geological interpretation map, scale 1: 2,400) - pdf file 8.8 MB
Structural
Interpretation
map, Keyes Mountain area
Outline location sketch map
- pdf file 0.5 MB
Location
of
the Keyes Mountain area, Vermont
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